α-Gel has a hydrate-type crystal structure, which is a lamellar structure. The intercellular lipid present in the horny layer (i.e., the outermost skin layer) generally has the α-gel structure, and prevents entry of outside substances into the skin as well as transepidermal water loss. Also, the intercellular lipid itself retains water, whereby the softness and smooth appearance of the skin can be maintained. In the skin, the horny layer retains water as bound water in an amount of about 33%. Studies have revealed that the intercellular lipid retains about 13% of the bound water (here, bound water is defined as water restrained by molecules forming the horny layer) (Non-Patent Document 1).
For example, Patent Document 1 discloses that an emulsion composition containing a ceramide and other ingredients forms an α-gel structure, to thereby enhance a moisturizing effect. When the skin surface is covered with such an α-gel, as covered with the intercellular lipid, the skin is expected to retain a sufficient amount of water under dry conditions, to thereby attain long-lasting moistness.
Meanwhile, low-molecular-weight compounds such as glycerin and amino acid as well as high-molecular-weight compounds such as hyaluronic acid are thought to have a high moisture retention property. However, since the water content of an aqueous solution of a single ingredient is generally limited to the bound water level, a preparation containing such an ingredient attains unsatisfactory water retention. In addition, although the aforementioned α-gel is expected to exhibit satisfactory water retention, solid fat that constitutes α-gel provides users a heavy feeling upon use. Thus, when the amount of α-gel is simply increased to enhance water retention, a good feeling upon use is impaired (e.g., sticky feeling upon use).